Liquid crystal display devices in widespread use have been in a backlight system where light emission is executed by illuminating a liquid crystal layer from the rear face. In this system, a backlight unit such as edge-lit backlight unit or a direct-lit backlight unit is mounted on the underside of the liquid crystal layer. As shown in FIG. 7a, such an edge-lit backlight unit 110 generally includes a reflection sheet 115 disposed on the front face of a top plate 116, an optical waveguide sheet 111 disposed on the front face of the reflection sheet 115, an optical sheet 112 disposed on the front face of the optical waveguide sheet 111, and a light source 117 that emits rays of light toward the end face of the optical waveguide sheet 111 (see Japanese Unexamined Patent Application, Publication No. 2010-177130). In the edge-lit backlight unit 110 shown in FIG. 7a, rays of light that have been emitted from the light source 117 and have entered the optical waveguide sheet 111 propagate through the optical waveguide sheet 111. A part of the propagating rays of light exit from the back face of the optical waveguide sheet 111, are reflected on the reflection sheet 115, and enter again the optical waveguide sheet 111.
In liquid crystal display devices having such a liquid crystal display unit, in order to enhance its portability and user-friendliness, a reduction in thickness and weight is required, leading to a requirement also for a reduction in thickness of the liquid crystal display unit. In particular, in ultrathin portable terminals in which the thickness of the thickest part of its housing is no greater than 21 mm, it is desired that the thickness of the liquid crystal display unit is about 4 mm to 5 mm, and thus, even further a reduction in thickness of the edge-lit backlight unit incorporated into the liquid crystal display unit has been desired.
In regard to the edge-lit backlight unit of such an ultrathin portable terminal, in addition to the edge-lit backlight unit having the reflection sheet 115 disposed on the back face of the optical waveguide sheet 111 shown in FIG. 7a, an edge-lit backlight unit is also proposed in which a reduction in thickness is attempted, as shown in FIG. 7b, by omitting the reflection sheet 115 shown in FIG. 7a. The edge-lit backlight unit 210 shown in FIG. 7b includes a metal top plate 216, an optical waveguide sheet 211 overlaid on the front face of the top plate 216, an optical sheet 212 overlaid on the front face of the optical waveguide sheet 211, and a light source 217 that emits rays of light toward the end face of the optical waveguide sheet 211. The front face of the top plate 216 is finished by polishing and functions as a reflection surface 216a. In this example, the rays of light that have been emitted from the light source 217 and have entered the optical waveguide sheet 211 propagate through the optical waveguide sheet 211, and a part of the propagating rays of light exit from the back face of the optical waveguide sheet 211, are reflected on the reflection surface 216a as the front face of the top plate 216, and enter again the optical waveguide sheet 211. Thus, in the edge-lit backlight unit 210 shown in FIG. 7b, the front face of the top plate 216 corresponds to the reflection surface 216a, and therefore the reflection surface 216a can serve as a substitute for the reflection sheet 115 shown in FIG. 7a. Therefore, such an edge-lit backlight unit 210 omits the reflection sheet 115, leading to a facilitation of the reduction in thickness. In addition, some edge-lit backlight units for such ultrathin portable terminals include an optical waveguide sheet (light guide film) having an average thickness of no greater than 600 μm, whereby a further reduction in thickness is achieved.